1. Field of the Invention
The present invention relates to a fuel cell system.
2. Description of the Related Art
An example of a fuel cell of the related art is formed by a fuel cell stack in which a plurality of unit cells, for example, several hundred unit cells are stacked. Each unit cell is formed by a layered body including a membrane electrode assembly (MEA) shaped like a flat plate and separators stacked on either side of the MEA. The MEA has a three-layer structure in which an electrolyte membrane formed of ion-exchange resin or the like is provided between a pair of electrodes serving as a positive electrode (air electrode, cathode) and a negative electrode (fuel electrode, anode). In such a fuel cell, for example, when fuel gas is run through a gas flow passage facing a gas diffusion electrode on the fuel electrode side and oxidant gas is run through a gas flow passage facing a gas diffusion electrode on the air electrode side, an electrochemical reaction occurs to generate power.
To stabilize the above-described electrochemical reaction and to maintain a high power generation efficiency, it is necessary to keep the electrolyte membrane in a water-saturated state so as to ensure the function of ion-exchange resin. For example, Japanese Unexamined Patent Application Publication No. 6-132038 discloses a fuel cell system using moisture exchange. In this fuel cell system, air sent from a blower is applied to a humidifying unit, and moisture contained in used air exhausted from a fuel cell stack (hereinafter sometimes referred to as cathode off-gas or off-gas) and used fuel gas (hereinafter sometimes referred to as anode off-gas or off-gas) is applied to the air, that is, moisture change is performed in the humidifying unit.
FIG. 2 illustrates an exemplary configuration of such a fuel cell system. As illustrated in FIG. 2, for example, when air is sucked from the atmosphere by a compressor 12 of a Lysholm type or a Roots type, oxidant gas is supplied to a cathode side of a fuel cell stack 10 through a flow passage 20 (oxygen in the air is used as oxidant). At the same time, off-gas containing water generated in the fuel cell is supplied to a humidifier 11 through a flow passage 21, and the humidifier 11 performs moisture exchange between the supply oxidant gas and the off-gas, so that the oxidant gas to be supplied to the fuel cell stack 10 can become moist.
However, the humidifier 11 includes a flat vapor permeation membrane having a large surface area and multiple hollow fibers such as to perform sufficient moisture exchange between the supply oxidant gas and the off-gas. Therefore, the size of the humidifier 11 itself is large. In particular, it is difficult to reduce the size of a system to be mounted in the vehicle.
Further, in a low air volume area of the compressor 12, air leaks from a rotor clearance, and the air supply efficiency of the compressor 12 becomes lower than in a high air volume area.
As measures against these problems, Japanese Unexamined Patent Application Publication No. 2000-294265 proposes a technique of injecting generated water in a fuel cell to a compressor provided on a supply oxidant-gas flow passage in order to avoid the decrease in efficiency of the compressor. According to this technique, an additive flow rate is given by injection of the generated water, and this can enhance the efficiency of the compressor in the low air volume area.